Method of coating a surface with silver from solution



United States Patent METHOD OF COATING A SURFACE WITH SILVER FROM SOLUTION Ernest H. Millard, Jr., Baltimore, Md., assignor to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Application August 18, 1954 Serial No. 450,783

4 Claims. (Cl. 117-35) This invention relates to a method of coating surfaces with an adherent film of metallic silver which is particularly adaptable to modern production line techniques. The art of applying silver films to various surfaces is relatively old, possibly originating in the observation of Liebig about 1835, that an aldehyde will reduce a warm ammoniacal silver oxide solution to precipitate metallic silver in a form which will adhere to and form a mirror on a glass surface. The classical method of silvering surfaces employing aldehyde, e. g., formaldehyde as reducing agents, is still widely used in small scale operations. Various modifications of the classical method have been used in recent years, c. g., other reducing agents such as glyoxal, Rochelle salt and the like have been used. The classical methods of silvering surfaces are not particularly adaptable to industrial operations, however, since it is difiicult to produce consistently satisfactory results with these methods. Reaction times ranging from 1 to 20 minutes are usual and depend upon the concentrations and temperatures of the silver containing and reducing solutions. Thus, it may be seen that it is difficult to accurately control the variables inherent in these methods making satisfactory reproducible results difficult to obtain, especially on large-scale operations.

An advance was made in the art with the introduction of spray techniques for the production of adherent silver films. In these techniques, two solutions, one of ammoniacal silver oxide, and the other a reducing agent, are simultaneously mixed and sprayed onto the surface to be coated. A typical method of this type is described in U. S. Patent No. 2,214,476 which discloses the use of hydrazine sulfate, hydrazine nitrate, and hydrazine hydrate as reducing agents. The recited hydrazine compounds reduce ammoniacal silver oxide solutions so rapidly that it is possible to form a silver film on an inclined surface by simultaneously spraying a silver-containing solution and a solution containing one of the reducing agents onto the surface which is then drained and quickly washed and dried. Although more satisfactory for industrial purposes than the classical silver-ing methods, the newer spray methods have the disadvantage that much silver is wasted since reduction takes place so rapidly that much of the silver thrown out of solution fails to adhere to the surface to be coated. This problem has been partially solved as suggested in U. S. Patent No. 2,367,903, by the addition of various inorganic salts, e. g., magnesium sulfate, sodium sulfate, potassium sulfate and ammonium sulfate to the reducing solution to modify the action of the hydrazine hydrate or salts. This process, however, fails to provide a completely satisfactory industrial method of applying silver films to surfaces since the concentration of the reagents and the contact time are critical and difficult to control.

-I have now discovered a new method of applying adherent films of metallic silver to surfaces which is particularly adaptable to large-scale industrial operations. My new method employs the spray techniques which are now widely used in the art and new reducing agents, i. e.,

,B-hydroxyethylhydrazine and its salts, which are peculiarly adaptable to the spray technique. I have found that chemical modification of the old hydrazine reducing agents by the insertion of a B-hydroxyethyl radical in the molecule produces reducing agents which are less susceptible to variations in operating conditions and techniques than known reducing agents and which consequently produce satisfactory results even when used in large-scale operations under a wide variety of conditions. The new reducing agents control the rate of reduction of silver to insure the adherence of substantially all of the precipitated silver to the surface to be coated. Further, when the new re ducing agents are employed, it is unnecessary to introduce to the treating solutions, additional components which must be maintained within critical concentration limits. Therefore, the new reducing agents are particularly adapted for use in largesc-ale industrial operations employing spray techniques, although they may be used with advantage in any of the processes of applying silver films and on any scale, large or small.

The surfaces to be silvered by the new method are prepared by standard techniques in the art, e. g., by treatment with dilute stannous chloride, and the usual precautions regarding cleanliness are observed. My method is adapted to the production of adherent films of metallic silver on non-conducting materials of all types, both transparent and opaque, including glass, synthetic resins such as polystyrene, polymethylmethacrylate, phemoi-aldehyde resins and cellulose esters and the like. Therefore, the new method provides for the production of mirrors by producing silver films on transparent substances and for various types of surface decorations by producing films on opaque substances. The films resulting from the process of my invention are of high quality, particularly in that they are homogeneous, of uniform thickness, and free from pinholes.

In general, to accomplish the purposes of my invention, it is necessary to employ an aqueous ammoniacal silvering solution containing ,B-hydroxyethylhydrazine or one of its salts as a reducing agent. These silvering solutions are ordinarily made up by dissolving silver nitrate in distilled water and converting the dissolved salt to the oxide by treatment with sutficient quantities of aqueous ammonium hydroxide. The dissolved silver oxide is then acted upon by my new reducing agents to produce metallic silver in a form which will adhere to the surfaces to be coated.

The treating solutions useful in my method of applying adherent silver films may be formulated in various ways. For example, a solution of silver nitrate containing about 1 to 2.5 percent of the salt by weight may be treated with 28 percent aqueous ammonia until the precipitate which forms initially just dissolves. A small amount of 2 percent silver nitrate solution is then added until the solution darkens, thus producing a silver oxide containing solution suitable for use in my invention. A reducing solution suitable for use with the above silver-containing solution may be prepared by dissolving B-hydroxyvethylhydrazine sulfate or the equivalent amount of the corresponding base or any other salt of the base in water to give a 1 to 2 percent solution. The solutions prepared in this way may be used in any of the standard silvering techniques but are employed with greatest advantage by substantially simultaneously mixing and spraying the two solutions in equal proportions onto the surface to be coated. In another modification of my invention one of the solutions may contain silver nitrate in water, while the second solution contains l3-hydroxyethylhydrazine or its salts and ammonium hydroxide in suitable proportions. In still another modification of the invention the reducing agent may be dissolved in one solution with the silver nitrate, and the second solution may contain the arnmonium'hydroxide. Small amounts of sodium hydroxide may also be incorporated in the treating solutions as is known in the art to accelerate the reaction but with the improved reducing agents of '-this invention an accelerator is seldom necessary.

The following example is for illustrative purposes only. It is :not to be construed as limiting the scope of my invention.

Example vI A silver-containing solution is prepared by dissolving 4 ounces of silver nitrate in 160 ounces of distilled water, adding 8 to 12 ounces of 28 percent aqueous ammonia slowly with stirring until the initially formed precipitate dissolves. A reducing solution is formed by dissolving 1.5 ounces of fl hydroxyethylhydrazine in 160 ounces of distilled water. The two solutions at ambient temperatures are then supplied at equal rates to a twin nozzle compressed air spray gun and discharged against a slanting surface of glass. The resulting metallic silver film is washed with water and dried to obtain an excellent mirror.

I claim:

1. A method of coating a surface with an adherent film of metallic silver which comprises substantially simultaneously mixing and spraying onto the surface to be coated two aqueous solutions; one solution comprising silver nitrate in a concentration of about 1 to 2.5 percent by weight and the other solution comprising a reducing "agent selected from the group consisting of fi-hydroxyethylhydrazine and its salts in a concentration of about 1 to 2 percent by weight; at least one of said solutions containing ammonium hydroxide.

is -B-hydroxyethylhydrazine.

3. A method of coating a surface with an adherent film of metallic silver which comprises substantially simultaneously mixing and spraying onto the surface to be coated two aqueous solutions; one solution comprising approximately 4 ounces of silver nitrate, approximately 8 to 12 ouncesof 28% aqueous-ammonia and approximately 160 ounces of water, the amount of ammonia being sufiicient to just-dissolvethe precipitate which forms, and the other solution comprising approximately 1.5 ounces of fl-hydroxyethylhydrazine in approximately 160 ounces of water.

4. A method of coating a surface with an adherent film of metallic silver which comprises substantially simultaneously mix-ing and spraying onto the surface to be coated two aqueous solutions; one solution comprising approximately 1.6 to 4 ounces of silver nitrate, suflicient aqueous ammonia to dissolve the precipitate which forms and approximately 160 ounces of water, and the other solution comprising approximately 1.5 to 3.2 ounces of B-hydroxyethylhydrazine in approximately 160 ounces of water.

References Cited in the file of this patent UNITED STATES PATENTS 2,214,476 Peacock Sept. 10, 1940 OTHER REFERENCES Benoit: Societe Chimique, ,pp. 708-715, 1939. 

1. A METHOD OF COATING A SURFACE WITH AN ADHERENT FILM OF METALLIC SILVER WHICH COMPRISES SUBSTANTIALLY SIMULTANEOUSLY MIXING AND SPRAYING ONTO THE SURFACE TO BE COATED TWO AQUEOUS SOLUTIONS; ONE SOLUTION COMPRISING SILVER NITRATE IN A CONCENTRATION OF ABOUT 1 TO 2.5 PERCENT BY WEIGHT AND THE OTHER SOLUTION COMPRISING A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF B-HYDROXYETHYLHYDRAZINE AND ITS SALTS IN A CONCENTRATION OF ABOUT 1 TO 2 PERCENT BY WEIGHT; AT LEAST ONE OF SAID SOLUTIONS CONTAINING AMMONIUM HYDROXIDE. 